Add Spoilers as per MSC2010

res/css/views/rooms/_EventTile.scss

@@ -296,6 +296,25 @@ div.mx_EventTile_notSent.mx_EventTile_redacted .mx_UnknownBody {
     overflow-y: hidden;
 }
 
+/* Spoiler stuff */
+.mx_EventTile_spoiler {
+    cursor: pointer;
+}
+
+.mx_EventTile_spoiler_reason {
+    color: $event-timestamp-color;
+    font-size: 11px;
+}
+
+.mx_EventTile_spoiler_content {
+    filter: blur(5px) saturate(0.1) sepia(1);
+    transition-duration: 0.5s;
+}
+
+.mx_EventTile_spoiler.visible > .mx_EventTile_spoiler_content {
+    filter: none;
+}
+
 .mx_EventTile_e2eIcon {
     display: block;
     position: absolute;

src/HtmlUtils.js

@@ -256,7 +256,7 @@ const sanitizeHtmlParams = {
     allowedAttributes: {
         // custom ones first:
         font: ['color', 'data-mx-bg-color', 'data-mx-color', 'style'], // custom to matrix
-        span: ['data-mx-bg-color', 'data-mx-color', 'style'], // custom to matrix
+        span: ['data-mx-bg-color', 'data-mx-color', 'data-mx-spoiler', 'style'], // custom to matrix
         a: ['href', 'name', 'target', 'rel'], // remote target: custom to matrix
         img: ['src', 'width', 'height', 'alt', 'title'],
         ol: ['start'],

src/components/views/elements/Spoiler.js

@@ -0,0 +1,35 @@
+'use strict';
+
+import React from 'react';
+
+module.exports = React.createClass({
+    displayName: 'Spoiler',
+
+    getInitialState() {
+        return {
+            visible: false,
+        };
+    },
+
+    toggleVisible(e) {
+        if (!this.state.visible) {
+            // we are un-blurring, we don't want this click to propagate to potential child pills
+            e.preventDefault();
+            e.stopPropagation();
+        }
+        this.setState({ visible: !this.state.visible });
+    },
+
+    render: function() {
+        const reason = this.props.reason ? (
+            <span className="mx_EventTile_spoiler_reason">{"(" + this.props.reason + ")"}</span>
+        ) : null;
+        return (
+            <span className={"mx_EventTile_spoiler" + (this.state.visible ? " visible" : "")} onClick={this.toggleVisible.bind(this)}>
+                { reason }
+                &nbsp;
+                <span className="mx_EventTile_spoiler_content" dangerouslySetInnerHTML={{ __html: this.props.contentHtml }} />
+            </span>
+        );
+    }
+})

src/components/views/messages/TextualBody.js

@@ -95,6 +95,8 @@ module.exports = React.createClass({
     },
 
     _applyFormatting() {
+        this.activateSpoilers(this.refs.content.children);
+
         // pillifyLinks BEFORE linkifyElement because plain room/user URLs in the composer
         // are still sent as plaintext URLs. If these are ever pillified in the composer,
         // we should be pillify them here by doing the linkifying BEFORE the pillifying.
@@ -183,6 +185,34 @@ module.exports = React.createClass({
         }
     },
 
+    activateSpoilers: function(nodes) {
+        let node = nodes[0];
+        while (node) {
+            if (node.tagName === "SPAN" && typeof node.getAttribute("data-mx-spoiler") === "string") {
+                const spoilerContainer = document.createElement('span');
+
+                const reason = node.getAttribute("data-mx-spoiler");
+                const Spoiler = sdk.getComponent('elements.Spoiler');
+                node.removeAttribute("data-mx-spoiler"); // we don't want to recurse
+                const spoiler = <Spoiler
+                    reason={reason}
+                    contentHtml={node.outerHTML}
+                />;
+
+                ReactDOM.render(spoiler, spoilerContainer);
+                node.parentNode.replaceChild(spoilerContainer, node);
+
+                node = spoilerContainer;
+            }
+
+            if (node.childNodes && node.childNodes.length) {
+                this.activateSpoilers(node.childNodes);
+            }
+
+            node = node.nextSibling;
+        }
+    },
+
     findLinks: function(nodes) {
         let links = [];